Why is urine yellow? Scientists pinpoint enzyme responsible for giving the bodily fluid its hue

Urine is a fundamental diagnostic tool, revealing a wide range of disorders; However, scientists have just unraveled one of its mysteries: how body fluid turns yellow.

A team of scientists from the University of Maryland and the US National Institutes of Health discovered that the tone comes from a molecule formed from bacteria in our digestive system.

It’s an enzyme called bilirubin reductase (BilR), and it was the missing piece of the century-and-a-half-old puzzle that explains why urine is yellow.

The team also found that almost all healthy adults have BilR in their digestive system, but many newborns and people with inflammatory bowel disease do not have it.

When blood cells reach the end of their life cycle at six months, they release an orange protein called bilirubin, which eventually converts to urobilin, the molecule responsible for coloring urine yellow.

Bilirubin is a waste product and is generally considered toxic by doctors.

Some investigation However, it suggests it could help patients tolerate organ transplants and possibly even treat conditions such as stroke and multiple sclerosis.

It passes into the digestive system, where most of it passes, but some is reabsorbed into the body.

The liver filters and absorbs as much of this excess as possible.

Hundreds of natural bacteria thrive in our digestive system and produce chemicals that help us break down food to access its nutrients and eliminate waste products like bilirubin.

Nine of these bacteria break down bilirubin, the authors of the new study found.

Three of them produce BilR, the enzyme that converts bilirubin into urobilinogen and stercobilinogen molecules, which can be easily excreted in urine and feces.

“Gut microbes encode the enzyme bilirubin reductase, which converts bilirubin into a colorless byproduct called urobilinogen,” the study’s senior author, Brantley Hall, an assistant professor of cell biology and molecular genetics at the University of Maryland, said in a paper. statement.

“Urobilinogen spontaneously breaks down into a molecule called urobilin, which is responsible for the yellow color we all know.”

Scientists have known about urobilin since it was first discovered in 1868, 156 years ago.

But they didn’t know about BilR, the enzyme that helps make urobilin.

The results appeared in the magazine. nature microbiology.

“Now that we have identified this enzyme, we can begin to investigate how bacteria in our gut affect circulating bilirubin levels and related health conditions, such as jaundice,” study co-author Xiaofang Jiang said in the release.

Jaundice, characterized by yellowing skin and eyes, occurs when the liver cannot eliminate excess bilirubin.

“This discovery lays the foundation for understanding the gut-liver axis,” Jiang said.

Scientists have increasingly discovered that the gut microbiome, the population of microbes that live in our digestive system, is linked to all kinds of health conditions, including depression, arthritis, and even aggression.

This new study adds another piece to the understanding of how the microbiome keeps us healthy.

“The multidisciplinary approach we were able to implement, thanks to collaboration between our laboratories, was key to solving the physiological puzzle of why our urine appears yellow,” Hall said. “It is the culmination of many years of work by our team and highlights yet another reason why our gut microbiome is so vital to human health.”